The present invention relates in general to modulator and demodulator circuits, and more particularly to reducing the phase error of quadrature signals in modulator and demodulator circuits.
Quadrature signals are commonly used in transmitters and receivers in wireless communications systems for modulating and demodulating information signals on a radio frequency carrier signal. They are used in such diverse applications as satellite receivers, cellular and cable telephone systems, global positioning systems and cable television set-top boxes. These applications operate at carrier frequencies between 500 megahertz and 3 gigahertz.
Two signals are in quadrature with respect to each other when they have the same frequency but are 90 degrees apart in phase. Accurate modulation or demodulation requires an in-phase signal and a quadrature signal having a constant amplitude and as small a phase error between it and the in-phase signal as possible. At higher frequencies, however, sufficient accuracy is more difficult to achieve because circuit parasitics and signal propagation delays give rise to phase errors and amplitude variations between the in-phase and quadrature signals that lower the performance of the modulator or demodulator circuits.
Several methods have been devised for generating a quadrature signal from an in-phase or reference signal. For example, one method uses complementary resistor-capacitor and capacitor-resistor phase shift networks to generate a quadrature signal from a reference signal. However, this method is not adequate for systems that must operate over a range of frequencies because it produces a quadrature signal which varies strongly with variations in the frequency of the in-phase signal.
Another method for generating quadrature signals uses an input signal having twice the frequency of the in-phase and quadrature signals. A digital divide-by-two frequency divider generates an in-phase signal from the input signal. The quadrature signal is generated by logically comparing the input and in-phase signals. This method is inadequate for high frequencies because it requires circuits to operate at twice the frequency of the in-phase signal. Another disadvantage is that the resulting quadrature signal does not have a fifty percent duty cycle so that additional circuitry is needed for restoring the quadrature signal so it has a fifty percent duty cycle.
There is a need for an apparatus and a method for reducing phase error in generating a quadrature signal from an in-phase or reference signal in order to increase the accuracy of modulators and demodulators operating at high frequencies. It would be a benefit if the method could generate a quadrature signal having a fifty percent duty cycle and constant amplitude over a range of frequencies without requiring additional circuitry.